Forward carrier recovery using forward error correction (fec) feedback

ABSTRACT

A system receive, from an optical receiver, a signal derived from a first optical signal and a second optical signal generated by a local oscillator, that includes a first component that is an in-phase component and a second component that is a quadrature phase component; filter the signal, using a filter, set to one or more configurations, to obtain one or more recovered signals, where each of the recovered signals includes a respective quantity of noise; perform forward error correction, on the recovered signals, to obtain one or more quantities of bit errors that correspond to the recovered signals; and process the signal using the filter set to a particular configuration, of the one or more configurations, that corresponds to a lowest quantity of bit errors of the one or more quantities of bit error.

BACKGROUND

Coherent optical receivers process traffic, in the form of opticalsignals, received from an optical network. Coherent optical receiversprocess the optical signals by performing operations on the opticalsignals, such as polarization beam splitting, demodulation,analogue-to-digital conversion, etc. The processing usually includescoherent processing using a local oscillator that is matched to acarrier frequency that is generated by a remote oscillator in an opticaltransmitter.

Coherent optical receivers use a carrier recovery technique, such asfeedforward carrier recovery (FFCR), for tracking phase differencebetween the remote and local oscillators. The difference in phase (e.g.,phase noise) may be associated with phase noise from the remoteoscillator, the local oscillator, and/or the optical path (e.g., opticalfiber) that interconnects the optical transmitter to the receiver.However, the performance, associated with the FFCR technique, issensitive to how well a filter, used by the coherent optical receiver tosmooth the recovered phase, is matched to the noise characteristics ofthe signal.

FFCR may, however, introduce cycle slips that are caused by a transientloss of phase lock, by a carrier recovery loop circuit, within thecoherent optical receiver. The cycle slips can cause bit errors to occurwhen processing the signal. While the errors may be correctable, thecycle slips may increase a quantity of risk, associated with anoccurrence of an uncorrectable frame and/or loss of data, as a result ofa cycle slip.

SUMMARY

According to one implementation, a method, performed by a device, isprovided. The method may include receiving, from an optical receiver, asignal, derived from a first optical signal and a second optical signalgenerated by a local oscillator, that includes a first component that isan in-phase component and a second component that is a quadrature phasecomponent; and filtering the signal, using a filter set to one or moreconfigurations, to obtain one or more recovered signals, where each ofthe one or more recovered signals include a respective quantity ofnoise. The method may also include performing forward error correction,on the one or more recovered signals, to obtain one or more quantitiesof bit errors that correspond to the one or more recovered signals; andprocessing the signal using the filter set to a particularconfiguration, of the one or more configurations, that corresponds to alowest quantity of bit errors of the one or more quantities of biterror.

According to another implementation, a device may include one or moreprocessors to receive, from an optical receiver, a signal derived from afirst optical signal combined with a second optical signal that isgenerated by a local oscillator; obtain, from the signal, a firstrecovered signal using a filter that is set up in a first configuration,where the first recovered signal includes a first quantity of noise; andidentify a first quantity of bit errors associated with the firstrecovered signal. The device may also include the one or more processorsto obtain, from the signal, a second recovered signal using the filterthat is set up in a second configuration, where the second recoveredsignal includes a second quantity of noise; identify a second quantityof bit errors associated with the second recovered signal; and processthe signal using the filter set up in the first configuration or thesecond configuration based on whether the first quantity of errors isgreater than the second quantity of errors.

According to a further implementation, a system may include one or moredevices to receive a signal, derived from a first optical signal and asecond optical signal, where the signal includes a first signalassociated with a first polarization and a second signal, associatedwith a second polarization that is orthogonal to the first polarization;retrieve a first index to generate a first weighted average, of opticalphase, associated with the first signal and a second weighted average,of the optical phase, associated with the second signal; filter thefirst signal and the second signal, using a filter that is set up basedon the first weighted average, to generate a first recovered signal, andthe second weighted average to generate a second recovered signal; andobtain a first and second bit error rate associated with the firstrecovered signal and the second recovered signal. The system may alsoinclude the one or more processors to retrieve a second index togenerate a third weighted average, of the optical phase, associated withthe first signal and a fourth weighted average, of the optical phase,associated with the second signal; filter the first signal and thesecond signal, using the filter that is set up based on the thirdweighted average, to generate a third recovered signal, and the fourthweighted average to generate the fourth recovered signal; obtain a thirdand fourth bit error rate associated with the third recovered signal andthe fourth recovered signal, respectively; and process the first andsecond signals using the filter set to the first configuration or thesecond configuration based on whether a sum of the first bit error rateand the second bit error rate is greater than another sum of the thirdbit error rate and the fourth bit error rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example network in which systems and/ormethods described herein may be implemented;

FIG. 2 is a diagram of example components of an optical transmitterdevice of FIG. 1;

FIG. 3 is a diagram of example components of an optical receiver deviceof FIG. 1;

FIG. 4 is a diagram of example components of a digital signal processingdevice of FIG. 1;

FIG. 5A is a diagram that illustrates example filter responses used bythe digital signal processing device of FIG. 1;

FIG. 5B is a diagram of example components of a filter, that is used bythe digital signal processing device of FIG. 1, to generate one or morefilter responses identified in FIG. 5A;

FIG. 6 is a flowchart of an example process for performing a carrierphase recovery operation according to an implementation describedherein;

FIG. 7 is a diagram that illustrates example contour plots associatedwith a carrier phase recovery operation, according to an implementationdescribed herein; and

FIG. 8 is a flowchart of an example process for performing anothercarrier phase recovery operation according to an implementationdescribed herein.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings.The same reference numbers in different drawings may identify the sameor similar elements. Also, the following detailed description does notlimit the implementations.

Systems and/or methods, described herein, may include a technique forreducing affects of phase noise, associated with a signal received froma coherent optical receiver, based on a quantity of bit errorsassociated with the signal. As described herein, a FFCR device maydynamically tune a filter to change a quantity of phase noise that isassociated with the signal. The FFCR device may communicate with aforward error correction (FEC) device to identify a manner in which aquantity of bit errors, associated with the signal, changes as afunction the changing quantity of phase noise. The FFCR device may tunethe filter in a manner that reduces the quantity of bit errorsassociated with the signal.

Systems and/or methods may enable the FFCR device to change a quantityof phase noise based on a weighted average of the phase noise associatedwith each constituent polarization component of the signal. The FFCRdevice may communicate with the FEC device to identify a manner in whicha quantity of bit errors, associated with the components, changes as afunction of the changing quantity of phase noise averaged between thecomponents. The FFCR device may process each of the components in amanner that reduces the quantity of bit errors associated with thecomponents.

The FFCR device may dynamically tune a first filter, for a firstcomponent, associated with a first polarization state, such as atransverse electric (TE) polarization state (e.g., that is parallel toan X-axis). The FFCR device may dynamically tune a second filter, for asecond component, associated with a second polarization state, such as atransverse magnetic (TM) polarization state (e.g., that is parallel to aY-axis, where the X- and Y-axes are orthogonal). The FFCR device may, inone example, process each of the first or second componentsindependently. In another example, the FFCR device may process thesignal based on a weighted average of the first component and/or thesecond component. The term average, as used herein, may include anumerical average, mean, median, mid-point, and/or a weighted average,weighted mean, etc.

FIG. 1 is a block diagram of an example network 100 in which systemsand/or methods described herein may be implemented. Network 100 mayinclude an optical transmitter 110, an optical receiver 120, a digitalsignal processing (DSP) device 130, an optical network path 140(hereinafter referred to as “optical path 140”), and an electricalnetwork path 150 (hereinafter referred to as “electrical path 150”).

The number of devices and/or paths, illustrated in FIG. 1, is providedfor explanatory purposes. In practice, there may be additional devicesand/or paths, fewer devices and/or paths, different devices and/orpaths, or differently arranged devices and/or paths than illustrated inFIG. 1. Also, in some implementations, one or more of the devices ofnetwork 100 may perform one or more functions described as beingperformed by another one or more of the devices of network 100. Forexample, functions attributable to optical receiver 120 could beperformed by DSP device 130 and/or by some other device.

Optical transmitter 110 may include one or more devices that generateand/or transmit an optical signal via optical path 140. Opticaltransmitter 110 may, in an example implementation, include one or morelasers that generate one or more optical signals. In another exampleimplementation, optical transmitter 110 may include a modulator thatmodulates the one or more optical signals based on input electricalsignals. In one example, optical transmitter 110 may modulate theoptical signals using phase shift keying (PSK) phase modulationtechniques. In yet another example implementation, optical transmitter110 may include a multiplexer to multiplex the modulated optical signals(e.g., using wavelength-division multiplexing) for transmission tooptical receiver 120 via optical path 140. Each of the optical signalsmay be associated with a different carrier wavelength as a result of themultiplexing.

Optical receiver 120 may include one or more devices that receive,convert, process, amplify, and/or demodulate electrical and/or opticalsignals in a manner described herein. In an example implementation,optical receiver 120 may be a coherent optical receiver that may receiveand/or process phase modulated optical signals. Optical receiver 120may, for example, receive an optical signal and may perform apolarization beam splitting operation to break the optical signal into afirst optical signal associated with a first polarization and/or asecond optical signal associated with a second polarization. The secondpolarization may, in one example, be orthogonal to the firstpolarization. Optical receiver 120 may process the first and/or secondoptical signals to generate a real component (e.g., an amplitude and/orin-phase (I) component) and/or an imaginary component (e.g., a phaseand/or quadrature (Q) component) for each of the first optical signaland/or the second optical signal. Optical receiver 120 may demodulatethe components, of the first and/or second optical signals, to createelectrical signals. Optical receiver 120 may transmit the components, aselectrical signals, to DSP device 130 via one or more electrical paths150.

DSP device 130 may include one or more devices that receive, process,and/or perform other operations on the electrical signals, received fromoptical receiver 120, in a manner described herein. DSP device 130 mayreceive electrical signals from optical receiver 120 via electricalpaths 150. DSP device 130 may perform a FFCR operation to identifyand/or modify a quantity of phase noise associated with the electricalsignals.

DSP device 130 may perform a carrier phase recovery operation to reduceaffects of phase noise in one or more of the optical signals. DSP 130may, for example, configure a filter, that is used to process thesignals when performing the FFCR operation, to determine a manner inwhich to minimize a quantity of bit errors (e.g., based on a bit errorrate) associated with the signals. DSP device 130 may perform otheroperations associated with forward error correction (FEC) on theelectrical signals. DSP device 130 may determine a quantity of biterrors associated with a filtered signal and may generate a bit errorrate associated with the filtered signal.

Optical path 140 may include a network path that is capable oftransporting an optical signal. In an example implementation, opticalpath 140 may be a fiber optic cable via which an optical signal istransported, from optical transmitter 110, to optical receiver 120.Optical path 140 may include non-linear characteristics that introduce aquantity of noise (e.g., phase noise, polarization modal dispersion,and/or other types of noise) into the optical signal when transportingthe optical signal to optical receiver 120.

Electrical path 150 may include a network path that is capable oftransporting an electrical and/or radio frequency (RF) signal. In anexample implementation, electrical path 150 may be a wired network path,wireless network path, and/or a combination of a wired and/or wirelessnetwork path. In another example implementation, electrical path 150 maybe a wave guide, a coaxial cable, and/or another network path that iscapable of transporting an electrical and/or RF signal. Electrical path150 may enable an electrical signal to be transported from opticalreceiver 120 to DSP device 130.

FIG. 2 is a diagram of example components of optical transmitter 110. Asillustrated in FIG. 2, optical transmitter 110 may include a collectionof components, such as a transmitter (Tx) 210, a modulator 220, and/or asource 225. Although FIG. 2 shows example components of opticaltransmitter 110, in other implementations, optical transmitter 110 maycontain fewer components, additional components, different components,or differently arranged components than depicted in FIG. 2. Furthermore,in some implementations, one or more of the components of opticaltransmitter 110 may perform one or more functions described as beingperformed by another one or more of the components of opticaltransmitter 110.

Transmitter 210 may include one or more components that are capable ofgenerating an optical signal that can be outputted to an optical fiber,such as optical path 140. In one implementation, transmitter 210 may bea laser that generates and/or transmit an optical signal at a particularwavelength and/or with a particular bandwidth, which may be tuned and/orcalibrated by a user of optical transmitter 110. Transmitter 210 may betuned to enable the wavelength to be changed in a manner that permitsthe optical signal to be sent over one or more channels associated withoptical path 140. Transmitter 210 may introduce a quantity of noise(e.g., phase noise and/or other types of noise) into the optical signalwhen generating and/or transmitting the optical signal.

Modulator 220 may include one or more components that are capable ofmodulating an optical signal received from transmitter 210. For example,modulator 220 may receive an optical signal from transmitter 210 and maymodulate the optical signal using an electrical signal or radiofrequency (RF) signal received from source 225. The electrical or RFsignal may include a train of pulses, which modulator 220 may use toswitch and/or modulate the optical signal to create a modulated signal.The modulated signal may include a train of pulses associated with aparticular power level, bandwidth, duty cycle, period, etc. In oneexample, the modulated signal may be phase modulated. The phasemodulated optical signal may, for example, be based on PSK-basedmodulation, such as differential PSK (DPSK), quadrature PSK (QPSK),binary (PSK), dual-polarization QPSK (DPQPSK), and/or higher-order PSKtechniques. Modulator 220 may send the modulated optical signal tooptical receiver 120 via optical path 140.

Source 225 may include one or more components that are capable ofgenerating an electrical or radio frequency (RF) signal that is used, bymodulator 220 to modulate the optical signal received from transmitter210. Source 225 may transmit the electrical or RF signal to modulator220.

FIG. 3 is a diagram of example components of optical receiver 120. Asillustrated in FIG. 3, optical receiver 120 may include a collection ofcomponents, such as a pair of polarization beam splitters (PBSs) 310-1and 310-2 (hereinafter referred to collectively as “PBSs 310” andindividually as “PBS 310”), a pair of polarization-diversity coherentreceivers (PDCR) 320-1 and 320-2 (hereinafter referred to collectivelyas “PDCRs 320” and individually as “PDCR 320”), a group of receivers(Rx) 330-1, . . . , 330-4 (hereinafter referred to collectively as“receivers 330” and individually as “receiver 330”), and a group ofanalog-to-digital converters (ADC) 340-1, . . . , 340-4 (hereinaftercollectively referred to as “ADCs 340” and individually as an “ADC340”).

Although FIG. 3 shows example components of optical receiver 120, inother implementations, optical receiver 120 may contain fewercomponents, additional components, different components, or differentlyarranged components than depicted in FIG. 3. Furthermore, in someimplementations, one or more of the components of optical receiver 120may perform one or more functions described as being performed byanother one or more of the components of optical receiver 120.

PBS 310 may include one or more devices that receive an optical signaland/or convert the optical signal into different component signals eachhaving a different polarization. For example, PBS 310-1 may receive anoptical signal (e.g., shown as S in FIG. 3) from optical transmitter 110via optical path 140. The optical signal may, for example, includeconstituent polarization components associated with two orthogonal axes(e.g., TE and TM)). PBS 310-1 may use the optical signal to generate afirst optical signal (e.g., shown as S_(X)) associated with a firstpolarization, such as a TE polarization state, that is parallel to oneof the axes (e.g., the X-axis). PBS 310-1 may also use the opticalsignal to generate a second optical signal (e.g., shown as S_(Y) in FIG.3) associated with a second polarization, such as a TM polarizationstate, that is parallel to another one of the axes (e.g., the Y-axis).PBS 310-1 may output, to PDCR 320-1, the first optical signal and/or thesecond optical signal.

In another example, PBS 310-2 may receive a reference optical signal(e.g., shown as LO in FIG. 3) from a local oscillator (LO). The localoscillator may, for example, be a laser that generates the referenceoptical signal that is associated with a first frequency and/or phasethat is approximately equal to a second frequency (sometimes referred toherein as a “carrier frequency”) and/or phase, associated with theoptical signal (e.g., S), respectively. A degree to which the firstphase differs from the second phase may be caused by phase noiseintroduced into the optical signal, by the optical transmitter 110and/or optical path 140, and/or into the reference optical signal by thelocal oscillator.

PBS 310-2 may use the reference optical signal to generate a firstreference signal (e.g., shown as LO_(X)) associated with the firstpolarization. PBS 310-2 may also use the reference optical signal togenerate a second reference signal (e.g., shown as LO_(Y) in FIG. 3)associated with the second polarization. PBS 310-2 may output, to PDCR320-2, the first and/or the second reference signal. In another example,PBS 310-2 may be a splitter (e.g., that is a different type of splitterthan the polarization beam splitter), that generates the first referencesignal and/or the second reference signal.

PDCR 320 may include one or more devices that receive an optical signalassociated with a single polarization component and process the opticalsignal to generate real and/or imaginary components associated with theoptical signal. In an example implementation, PDCR 320 may be a 90degree hybrid device that combines a reference signal with a receivedoptical signal to generate real and/or imaginary components of thereceived optical signal and/or the reference signal.

PDCR 320-1 may, for example, receive the first optical signal (e.g.,S_(X)) from PBS 310-1 and/or the first reference signal (LO_(X)) fromPBS 310-2. PDCR 320-1 may mix the first reference signal with the firstoptical signal to generate real and imaginary components of the firstreference signal and/or the first optical signal. For example, PDCR320-1 may generate a real component of the first reference signal (e.g.,Re(LO)_(X)) and/or the first optical signal (e.g., shown as Re(S)_(X),where Re represents the real component). PDCR 320-1 may generate animaginary component of the first reference signal (e.g., Im(LO)_(Y))and/or the first optical signal (e.g., shown as Im(S)_(Y), where Imrepresents the imaginary component).

PDCR 320-2 may receive the second optical signal (e.g., S_(Y)) from PBS310-1 and/or the second reference signal (LO_(Y)) from PBS 310-2. PDCR320-2 may mix the second reference signal with the second optical signalto generate real and imaginary components of the second reference signaland/or the second optical signal. For example, PDCR 320-2 may generate areal component of the second reference signal (e.g., Re(LO)_(Y)) and/orthe second optical signal (e.g., shown as Re(S)_(Y)). PDCR 320-2 maygenerate an imaginary component of the second reference signal (e.g.,Im(LO)_(Y)) and/or the second optical signal (e.g., shown as Im(S)_(Y)).

PDCRs 320 may output the real and/or imaginary components of the firstand/or second optical signals, and/or the first and/or second referencesignals, to receivers 330. The real and/or imaginary components of thefirst and/or second optical signals, combined with the first and/orsecond reference signals, may be transmitted to receivers 330 via one ormore channels associated with optical receiver 120.

Receiver 330 may include one or more devices that receive an opticalsignal and convert the optical signal into an electrical signal.Receiver 330 may, in an example implementation, be a photo diode thatreceives, as input, the optical signal and generates, as output, anelectrical signal based on the received optical signal.

Receiver 330-1 may, for example, receive a real component of the firstoptical signal (e.g., Re(S)_(X)) and/or first reference signal (e.g.,Re(LO)_(X)) and may convert the real component, of the first opticalsignal and/or first reference signal, to a first electrical signal(e.g., shown as Re(S·LO)_(X)). Receiver 330-1 may transmit the firstelectrical signal to ADC 340-1. Receiver 330-2 may, in another example,receive an imaginary component of the first optical signal (e.g.,Im(S)_(X)) and/or the first reference signal (e.g., Im(LO)_(X)), and mayconvert the imaginary component, of the first optical signal and/or thefirst reference signal, to a second electrical signal (e.g., shown asIm(S·LO)_(X)). Receiver 330-2 may transmit the second electrical signalto ADC 340-2.

Receiver 330-3 may, in another example, receive a real component of thesecond optical signal (e.g., Re(S)_(Y)) and/or the second referencesignal (e.g., Re(LO)_(Y)) and may convert the real component, of thesecond optical signal and/or the second reference signal, to a thirdelectrical signal (e.g., shown as Re(S·LO)_(Y)). Receiver 330-3 maytransmit the third electrical signal to ADC 340-3. Receiver 330-4 may,in yet another example, receive an imaginary component of the secondoptical signal (e.g., Im(S)_(Y)) and/or the second reference signal(e.g., Im(LO)_(Y)), and may convert the imaginary component, of thesecond optical signal and/or the second reference signal, to a fourthelectrical signal (e.g., shown as Im(S·LO)_(Y)). Receiver 330-4 maytransmit the fourth electrical signal to ADC 340-4.

ADC 340 may include one or more devices that receive and/or process anelectrical signal to convert the electrical signal to a digitalelectrical signal. ADC 340 may, in an example implementation, convert anelectrical signal, received from receiver 330, to a digital electricalsignal for transmission to DSP device 130 via electrical path 150. ADC340 may, for example, sample an incoming electrical signal at a samplingrate that is greater than a threshold. The threshold may correspond to aNyquist sampling rate that is greater than two times a bandwidthassociated with the incoming electrical signal. ADC 340 may use thesampled signal to generate the digital electrical signal.

FIG. 4 is a diagram of example components of a DSP device 130. Asillustrated in FIG. 4, DSP device 130 may include a collection ofcomponents, such as a feedforward carrier recovery (FFCR) processor 410and a forward error correction (FEC) processor 420. Although FIG. 4shows example components of DSP device 130, in other implementations,DSP device 130 may contain fewer components, additional components,different components, or differently arranged components than depictedin FIG. 4. Furthermore, in some implementations, one or more of thecomponents of DSP device 130 may perform one or more functions describedas being performed by another one or more of the components of DSPdevice 130.

DSP device 130 may perform operations to correct or adjust bandwidth,associated with RF signals to conform to some bandwidth threshold. Inanother example, DSP device 130 may perform operations to correct oradjust quadrature angle error, associated with optical signals, toconform to some error threshold. In yet another example, DSP device 130may identify and/or remedy conditions (e.g., associated with opticalchannel conditions, transmit conditions, etc.) such as limited opticalbandwidth, polarization rotation, polarization mode dispersion,polarization dependent loss, or chromatic dispersion, etc.

FFCR processor 410 may include one or more components that that receive,process, and/or perform operations to identify a difference in phasebetween electrical signals and/or determine a quantity of phase noiseassociated with the electrical signals. In an example implementation,FFCR processor 410 may perform an FFCR operation to identify a quantityof phase noise associated with an electrical signal received fromoptical receiver 120. The identified quantity of phase noise mayrepresent a first quantity of phase noise associated with a remoteoscillator (e.g., such as a first laser associated with opticaltransmitter 110); a second quantity of phase noise caused by opticalpath 140 (e.g., from material defects and/or optical non-linearities ina fiber optic cable); a third quantity of phase noise associated with alocal oscillator (e.g., such as a second laser associated with opticalreceiver 120); and/or another quantity of phase noise (e.g., from othercomponents of optical transmitter 110, optical path 140 and/or opticalreceiver 130).

FFCR processor 410 may perform a carrier phase recovery operation toreduce a quantity of bit errors associated with a signal. FFCR processor410 may, for example, process a first signal received from opticalreceiver 120. For example, FFCR processor 410 may combine the firstelectrical signal (e.g., Re(S·LO)_(X)) and the second electrical signal(e.g., Im(S·LO)_(X)) to create a first signal with a first polarizationstate (e.g., that is parallel to an X-axis). The first signal with thefirst polarization state may be analyzed to estimate a first opticalphase (e.g., φX) on a symbol-by-symbol basis. The optical phase mayinclude a quantity of phase noise. FFCR processor 410 may use a filterto process optical phase to obtain the first estimated carrier phase(e.g., θX) associated with the first signal. The processing may smooththe optical phase by reducing a rate at which the optical phase changesas a function of time and/or on a per-symbol basis. In another example,FFCR processor 410 may combine the third electrical signal (e.g.,Re(S·LO)_(Y)) and the fourth electrical signal (e.g., Im(S·LO)_(Y)) tocreate a second signal with a second polarization state (e.g., that isparallel to a Y-axis). FFCR processor 410 may repeat the processing forthe second signal (to estimate a second optical phase (e.g., φY) and/ora second estimated carrier phase (e.g., θY).

The estimated carrier phase may represent a phase difference between aphase associated with transmitted optical signal (e.g., from transmitter210) and another phase associated with the LO oscillator. The filter mayuse one or more tap weights, to be described in greater detail withregard to FIG. 5A, to set up the filter in various configurations. Theestimated carrier phase may then be removed from the first signal togenerate a carrier recovered signal. FFCR processor 410 may, forexample, use an adaptation application to set up the filter, in a firstconfiguration, to generate a first carrier recovered signal (e.g., S1_(X)) that is transmitted to FEC processor 420. The first carrierrecovery signal may be corrupted by a first quantity of noise (e.g.,N₁), which may be detected as a first quantity of bit errors (e.g., biterror rate (BER₁)) by FEC processor 420. FEC processor 420 may correctthe first quantity of bit errors provided the first quantity of biterrors does not exceed a correction capacity threshold of FEC processor420.

The adaptation application may, in another example, set up the filter,in a second configuration, to generate a second carrier recovered signal(e.g., S2 _(X)). The second carrier recovery signal may be corrupted bya second quantity of noise (e.g., N₂), which may be detected as a secondquantity of bit errors (e.g., BER₂) by FEC processor 420.

The adaptation application may compare the first quantity of bit errors(e.g., BER₁) and the second quantity of bit errors (e.g., BER₂). If, forexample, the first quantity of bit errors is less than the secondquantity of bit errors (e.g., BER₁<BER₂), the adaptation application maycause FFCR processor 410 to process the first signal using the filterthat is set up in the first configuration. If, however, the firstquantity of bit errors is not less than the second quantity of biterrors (e.g., BER₁≧BER₂), then the adaptation application may cause FFCRprocessor 410 to process the first signal using the filter that is setup in the second configuration.

The FEC operations may be performed concurrently (e.g., within a periodof time) and/or at non-concurrently (e.g., at different periods oftime). When performing the FEC operations non-concurrently, FECprocessor 420 may measure the first and second quantities of bit errorat different points in time, corresponding to different filter shapes,chosen at the different points in time.

FFCR processor 410 may perform operations, in a manner similar to thatdescribed above, on a second signal that is associated with a secondpolarization state (e.g., that is parallel to a Y-axis, where the Y-axisis orthogonal to the X-axis). The adaptation application may, forexample, set up the filter, in a third configuration, to generate athird carrier recovered signal (e.g., S3 _(Y)). The third carrierrecovery signal may be corrupted by a third quantity of noise (e.g.,N₃), which may be detected as a third quantity of bit errors (e.g.,BER₃) by the FEC processor 420.

FFCR processor 410 may repeat the process, described above, using thefilter set up in a fourth configuration, to generate a fourth carrierrecovered signal (e.g., S4 _(Y)). The fourth carrier recovery signal maybe corrupted by a fourth quantity of noise (e.g., N₄), which may bedetected as a fourth quantity of bit errors (e.g., BER₄) by FECprocessor 420. FFCR processor 410 may process the second signal usingthe filter, set up in the third configuration when the third quantity ofbit errors is greater than the fourth quantity of bit errors (e.g.,BER₃≧BER₄). FFCR processor 410 may process the second signal using thefilter, set up in the fourth configuration when the third quantity ofbit errors is not greater than the fourth quantity of bit errors (e.g.,BER₃<BER₄).

The source of phase noise in optical systems may be correlated, betweenpolarization states (e.g., associated with the first signal and/or thesecond signal). The correlation may occur due to the first and/or secondsignal originating from a same source laser (e.g., associated withoptical transmitter 110), a same receive laser (e.g., associated withoptical receiver 120), and/or under the influence of fibernonlinearities (e.g., associated with optical path 140), which impairboth polarization states. In this case FFCR processor 410 may processthe first signal and/or the second signal, such that the optical phase,on a symbol-by-symbol basis, can be averaged between polarizations. Theaveraged optical phase may be filtered (e.g., smoothed as a function oftime) in a manner similar to that described above.

The averaging may, for example, be performed using weighting parameter(e.g., A and B, where A+B≅1). The averaging may be a weighted average ofa portion of the first optical phase (e.g., φX) and another portion ofthe second optical phase (e.g., φY) to create a weighted average of thefirst and second optical phase. For example, the weighting parameter maybe set to a first value (e.g., A=½, where B=1−½=½), that apportionsone-half of the average optical phase to the first estimated carrierphase (e.g., θX) and one-half of the average optical phase to the secondestimated carrier phase (e.g., θY). In another example, the weightingparameter may be set to a second value (e.g., A=0) that may cause thefirst and second polarizations to operate independently by notapportioning any of the average carrier phase to the first or secondestimated carrier phase.

In general, a quantity of phase applied to the first estimated carrier(e.g., OX) may be determined by applying the filter, in a manner similarto that described above, to the first weighted average of the opticalphase. (e.g., φ1≅AφY+BφX, where φ1 is the first weighted average of theoptical phase). In another example, another quantity of phase applied tothe second estimated carrier phase (e.g., θY) may be determined byapplying the smoothing filter to the second weighted average of theoptical phase (e.g., φ2≅AφX+BφY, where φ2 is the second weighted averageof the optical phase). The first and second estimated carrier phase maybe applied to the first and second signals (e.g., associated with thefirst and second polarizations) to generate the carrier recoveredsignals.

FFCR processor 410 may use an index (hereinafter referred to as an “XYaveraging index”) to specify the manner that the weighted averages areto be set. For example, a maximum XY averaging index may correspond toequal weighting (e.g., when A and B are approximately equal, such aswhen A=0.5 and B=0.5). In another example, a minimum XY averaging indexmay correspond to no averaging between the first and/or second signals(e.g., when A≅0 and B≅1), such as in the examples described above, wherethe first signal and/or the second signal are processed independently.In yet another example, an XY averaging index, between the minimum andmaximum XY averaging indexes, may correspond to a portion of the firstand third quantities of phase noise (e.g., where A=0.3, 0.25, etc.)being associated with the first signal and another portion of the firstand third quantities of phase noise (e.g., where B=0.7, 0.75, etc.).

FFCR processor 410 may, in one example, use a first XY averaging indexthat causes the first weighting factor and/or the second weightingfactor to be set to predetermined values (e.g., A1 and/or B1,respectively). FFCR processor 410 may generate fifth and/or sixthcarrier recovered signals (e.g., S5 _(X) and/or S6 _(Y), respectively).The fifth and/or sixth carrier recovered signals may be corrupted bysome combination of noise N₅ and N₆, which may be detected, as a fifthand/or sixth quantity of bit errors (e.g., BER₅ and/or BER_(G),respectively), by the FEC processor 420.

FFCR processor 410 may, in another example, use a second XY averagingindex that causes the first weighting factor and/or the second weightingfactor to be set to other values (e.g., A2 and/or B2, respectively) thatare different than the predetermined values. FFCR processor 410 maygenerate seventh and/or eighth carrier recovered signals (e.g., S7 _(X)and/or S8 _(Y), respectively). The seventh and/or eight carrier recoverysignals may be corrupted by some other combination of noise (e.g., N₇and N₈), which may be detected, as a seventh and/or eighth quantity ofbit errors (e.g., BER₇ and BER₈, respectively), by the FEC processor420.

Based on a determination that a sum of the fifth and sixth quantities ofbit errors (e.g., BER₅+BER₆) is less than another sum of the seventh andeighth quantities of bit errors (e.g., BER₇+BER₈), FFCR processor 410may select the first XY averaging index for continuing to generate thecarrier recovered signals. Based on a determination that the sum of thefifth and sixth quantities of bit errors is not less than the sum of theseventh and eighth quantities of bit errors, FFCR processor 410 mayselect the second XY averaging index for continuing to generate thecarrier recovered signals.

FEC processor 420 may include one or more components that receive and/orprocess signals to identify errors associated with the signals and/or toperform an error correction operation on the signals. FEC processor 420may, in an example implementation, perform a forward error correctionoperation on signals, received from FFCR processor 410, to identifyand/or correct bit errors associated with the signal. FEC processor 420may, in another example, use an iterative forward error correctionprocess to identify and/or correct bit errors associated with thesignals. The iterative process may permit FEC processor 420 to identifyand/or correct additional errors with each successive iteration. FECprocessor 420 may perform the iterative process until a measure of thequantity of bit errors per bit (e.g., a bit error rate), is less than athreshold.

FEC processor 420 may, in another example, send a measure of thequantity of bit errors (e.g., shown as BER_(N) and/or BER_(T) in FIG.4), associated with the signals, to FFCR processor 410. The measure ofthe quantity of bit errors may, in one example, be sent to FFCRprocessor 410 via a control-based feedback loop and/or network path.Sending the measure of the quantity of bit errors, to FFCR processor410, may enable FFCR processor 410 to perform a carrier phase recoveryoperation to reduce a quantity of phase noise associated with thesignals.

The measure of the quantity of bit errors may, in one example, beobtained prior to performing the forward error correction operation onthe signals (sometimes referred to as a pre-FEC bit error rate). Inanother example, the measure of the quantity of bit errors may beobtained during the forward error correction operation, such as after afirst iteration, a second iteration, etc. Selecting different metricsfor optimizing the carrier recovery may yield optimizations of thelikelihood of having uncorrectable frames, which, depending on a type ofFEC operation being performed, may not have a unique correlation withthe pre-FEC error rate. Obtaining a pre-FEC error rate may enable theFFCR operation, performed by FFCR processor 410, to be performed withina first period of time. The first period of time may be less than asecond period of time associated with performing the FFCR operationbased on a measure of the quantity of bit errors obtained during one ormore iterations associated with the forward error correction operation.

FIG. 5A is a diagram that illustrates example filter response 500 usedby DSP device 130. As illustrated in FIG. 5A, filter response 500 mayinclude a group of tap weight configurations 505-1, . . . , 505-P (whereP≧1) (hereinafter collectively referred to as “tap weight configurations505” and individually as “tap weight configuration 505”).

Tap weight configuration 505 may represent a configuration associatedwith a filter that is used by FFCR processor 410 to perform FFCRoperations and/or carrier phase recovery operations. An adaptationapplication, hosted by FFCR processor 410, may use informationassociated with one or more tap weight configurations 505 to process aportion of signals received from optical receiver 120. The portions ofthe signals may correspond to a quantity of optical phase associatedwith the signals received from optical receiver 120. Each tap weightconfiguration 505 may include a tap weight axis 510, a bit axis 515, acollection of tap weights 520-1, . . . , 520-Q (where Q≧1) (hereinafterreferred to collectively as “tap weights 520” and individually as “tapweight 520”), and tap weight index 525 (hereinafter referred to as“index 525”).

Tap weight axis 510 may identify a range of tap weight values. The rangeof tap weight values may identify a range of values, such as from aminimum value (e.g., of 0 and/or some other minimum value) to a maximumvalue (e.g., of 1 and/or some other maximum value), to which tap weight520 can be set by the adaptation application. Bit axis 515 may identifya range of bit positions that correspond to bits being sampled by asignal being processed via the filter. For example, negative bitpositions (e.g., from −9 to −1) may correspond to bits arriving at alater point in time relative to positive bit positions (e.g., from 1 to9). Bit axis 515 illustrates bit positions of −9 to +9 for explanatorypurposes. In other implementations, bit axis 515 may include anotherrange of bit positions that are different than the range of bitpositions included in FIG. 5A.

Tap weight 520 may represent a manner in which a bit, associated with asignal, is to be sampled. A bit that is sampled by tap weight 520 may bebased on a product of a value associated with the tap weight and anamplitude associated with the bit (e.g., a voltage level, a power level,a digital representation of voltage or power, etc.). For example, afirst tap weight 520 of zero (e.g., tap weight 520-1) may indicate thata bit that corresponds to the first tap weight 520 may not be sampled bythe filter. A bit that is not sampled, may not be included in a portionof the signal that is sampled by tap weights 520 that are greater thanzero. In another example, a second tap weight 520 of one (e.g., tapweight 520-Q) may indicate that a bit that corresponds to the second tapweight 520 may be fully sampled by the filter (e.g., based on a productof the value associated with tap weight 520-Q×amplitude of the bit). Inyet another example, other tap weights 520 (e.g., tap weights 520-2, . .. , 520-4) may be used to sample bits that may result in less than afully sampled bit.

Index 525 may represent a value that corresponds to a filterconfiguration 505 based on a manner in which tap weights 520 are set.For example, for tap weight configuration 505-4, N index value 525 maycorrespond to a value (e.g., 4.0) associated with tap weightconfiguration 505-4. The adaptation application may use tap weightconfigurations 505, associated with index 525 that is less than athreshold (e.g., such as tap weight configurations 505-1 through 505-4),to sample signals associated with a high noise level (e.g., a quantityof phase noise that is greater than a threshold). In another example,the adaptation application may use other tap weight configurations 505,associated with index 525 that is not less than a threshold (e.g., suchas tap weight configurations 505-11 through 505-P), to sample signalsassociated with a low noise level (e.g., a quantity of phase noise thatis not greater than the threshold).

FIG. 5B is a diagram of example components of a filter 550, that is usedby DSP 130, to generate one or more filter responses identified in FIG.5A. For example, FFCR processor 410 may use filter 550 to filtersignals, that correspond to an estimated quantity of optical phase(e.g., φ), included within a signal received from optical receiver 120(e.g., Re(S·LO)_(X)+Im(S·LO)_(X) and/or Re(S·LO)_(Y)+Im(S·LO)_(Y)).Filter 550 may output a signal that includes an estimated carrier phase(e.g., 8) associated with the signals received from optical receiver120. In an example implementation, filter 550 may be a finite impulseresponse (FIR) filter. As shown in FIG. 5B, filter 550 may include acollection of components, such as a group of multipliers 555-1, . . .555-J (where J≧1) (hereinafter referred to collectively as “multipliers555” and individually as “multiplier 555”), and a summing component 558.Although FIG. 5B shows example components of filter 550, in otherimplementations, filter 550 may contain fewer components, additionalcomponents, different components, or differently arranged componentsthan depicted in FIG. 5B. Furthermore, in some implementations, one ormore of the components of filter 550 may perform one or more functionsdescribed as being performed by another one or more of the components offilter 550.

Multiplier 555 may include one or more components that are capable ofreceiving, processing, and/or combining two or more signals. Forexample, multiplier 555 may receive two or more signals and may combinethe two or more signals into a combined signal that is outputted tosumming component 558. Multiplier 555 may combine the signals bymultiplying one of the signals by another one of the signals to createthe combined signal.

Multiplier 555-1 may, for example, receive a first signal (e.g., tapweight coefficient 560-1) that corresponds to a first tap weight 520(e.g., 520-1) of FIG. 5B. Multiplier 555-1 may receive a second signal(e.g., optical phase φ 565-1) associated with an estimated quantity ofcarrier phase φ obtained, by FFCR processor 410, from the signalreceived from optical receiver 120. The second signal may correspond toa first bit associated with the second signal. Multiplier 555-1 maycombine the first signal and the second signal by multiplying the firstsignal by the second signal to create a first combined signal 570-1.Multiplier 555-1 may output the first combined signal 570-1 to summingcomponent 558.

In another example, multiplier 555-2 may receive another first signal(e.g., tap weight coefficient 560-2) that corresponds to a second tapweight 520 (e.g., 520-2) of FIG. 5B. Multiplier 555-2 may receiveanother second signal (e.g., optical phase φ 565-2) associated with theestimated quantity of carrier phase φ. The second signal may correspondto a second bit associated with the second signal. Multiplier 555-2 maymultiply the other first signal by the other second signal to create asecond combined signal 570-2. Multiplier 555-2 may output the secondcombined signal 570-2 to summing component 558.

Filter 550 may include a multiplier 555 for each tap weight identifiedin FIG. 5A. Each of multipliers 555 may combine a respective firstsignal 560 and a respective second 565 to generate a respective combinedsignal 570 that is outputted summing component 558.

Summing component 558 may include one or more components that arecapable of receiving signals and/or summing the received signals. Forexample, summing component 558 may receive two or more combined signals570, from multipliers 555, and may output another signal based on thecombined signals 570. The outputted signal may represent a sum ofcombined signals 570. More particularly, the outputted signal mayrepresent a filtered signal that is based on a manner in which tapweights 520 (e.g., corresponding to tap weight coefficients 560) sampleda portion of bits associated with optical phase φ 565. The filteredsignal, that is outputted by summing component 558, may represent anestimated carrier phase θ associated with the signal received fromoptical receiver 120.

FIG. 6 is a flowchart of an example process for recovering carrier phaseaccording to an implementation described herein. In one exampleimplementation, process 600 may be performed by FFCR processor 410and/or FEC processor 420. In another example implementation, some or allof process 600 may be performed by a device or collection of devicesseparate from, or in combination with, FFCR processor 410 and/or FECprocessor 420.

As shown in FIG. 6, process 600 may include detecting a signal from anoptical receiver and obtaining a default index value (block 605). Forexample, FFCR processor 410 may receive a signal from optical receiver120 and may obtain, from a memory associated with FFCR processor 410, adefault index value (e.g., default index 525) with which to recover thecarrier phase. The signal may be associated with a first polarizationstate (e.g., that is parallel to an X-axis). In another exampleimplementation, the signal may be associated with a second polarizationstate (e.g., that is parallel to an Y-axis, where the Y-axis may beorthogonal to the X-axis). In yet another example implementation, thesignal may be associated with a third polarization state (e.g., adual-polarization state that includes components associated with theX-axis and/or the Y-axis).

As also shown in FIG. 6, process 600 may include setting first tapweights based on a first value that is greater than the index value(block 610) and sampling the signal using the first tap weights (block615). For example, FFCR processor 410 may identify a first value that isgreater than the default index 525 by a predetermined quantity. Thepredetermined quantity may, in one example, be a constant that isgreater than zero (e.g., 0.25, 0.5, 1.0, etc.). In another example, thepredetermined quantity may be a portion (e.g., such as a percentage) ofindex 525 that is greater than zero (e.g., 10 percent, 20 percent,etc.).

FFCR processor 410 may set first tap weights 520, associated with tapweight configuration 505, to correspond to the first value. If, forexample, the first value is approximately equal to six (e.g., based on asum of a default index 525 of 5.5 and the predetermined quantity of0.5), then FFCR processor 410 may set the first tap weights 520 tocorrespond to tap weight configuration 505-6 associated with index 525of 6.0 (e.g., as shown in FIG. 5A). FFCR processor 410 may process the(e.g., by smoothing a rate of change associated with the optical phaseas a function of time) signal, in a manner similar to that describedabove with respect to FIG. 4, using the filter that is configured basedon the first tap weights 520 to obtain a first estimated carrier phase(e.g., φX or φY).

As further shown in FIG. 6, process 600 may include obtaining a firstquantity of bit errors associated with the sampled signal using thefirst tap weights (block 620). For example, FFCR processor 410 may send,to FEC processor 420, a request to obtain a first quantity of bit errors(e.g., BER₁) associated with the first estimated carrier phase. Therequest may include the first estimated carrier phase that was processedbased on the first tap weights. FEC processor 420 may receive therequest and may perform an operation to identify a quantity of biterrors associated with first estimated carrier phase. FEC processor 420may, in one example, identify the quantity of bit errors prior toperforming a forward error correction operation on the first estimatedcarrier phase. FEC processor 420 may, in another example, identify thequantity of bit errors during a period of time when the forward errorcorrection is being performed (e.g., after a first forward errorcorrection iteration and/or another forward error correction iteration).FEC processor 420 may send the identified quantity of bit errors, toFFCR processor 410, as the first quantity of bit errors (e.g., BER₁).FFCR processor 410 may receive the first quantity of bit errors.

As yet further shown in FIG. 6, process 600 may include setting secondtap weights based on a second value that is less than the index value(block 625) and sampling the signal using the second tap weights (block630). For example, FFCR processor 410 may identify a second value thatis less than the default index 525 by the predetermined quantity oranother predetermined quantity that is different than the predeterminedquantity.

FFCR processor 410 may set second tap weights 520, associated with tapweight configuration 505, to correspond to the second value. If, forexample, the second value is approximately equal to five (e.g., based ona difference between the default index 525 of 5.5 and the predeterminedquantity of 0.5), then FFCR processor 410 may set the second tap weights520 to correspond to tap weight configuration 505-5 associated withindex 525 of 5.0 (e.g., as shown in FIG. 5A). FFCR processor 410 mayprocess the signal using the filter that is configured based on thesecond tap weights 520 to obtain a second estimated carrier phase.

As still further shown in FIG. 6, process 600 may include obtaining asecond quantity of bit errors associated with the sampled signal usingthe second tap weights (block 635). For example, FFCR processor 410 maysend, to FEC processor 420, a request to obtain a second quantity of biterrors (e.g., BER₂) associated with the second estimated carrier phase.The request may include the second portion of the signal that wassampled based on the second tap weights. FEC processor 420 may receivethe request and may, in a manner similar to that described above (e.g.,with respect to block 620), perform an operation to identify a quantityof bit errors associated with second estimated carrier phase. FECprocessor 420 may send the identified quantity of bit errors, to FFCRprocessor 410, as the second quantity of bit errors (e.g., BER₂). FFCRprocessor 410 may receive the second quantity of bit errors.

As also shown in FIG. 6, if the first quantity of bit errors is greaterthan the second quantity of bit errors (block 640—YES), then process 600may include processing the signal using the second tap weights (block645). For example, FFCR processor 410 may compare the first quantity ofbit errors to the second quantity of bit errors and may determine thatthe first quantity of bit errors is greater than the second quantity ofbit errors. Based on the determination that the first quantity of biterrors is greater than the second quantity of bit errors, FFCR processor410 may process the signal using the filter that is set up based on thesecond tap weights 520.

As further shown in FIG. 6, process 600 may include decreasing the indexvalue based on a difference between the first and second quantities ofbit errors (block 650) and setting the index value equal to a minimumindex value if the decreased index value is less than the minimum indexvalue (block 655). For example, FFCR processor 410 may identify a firstdifference between the first quantity of bit errors (e.g., BER₁) and thesecond quantity of bit errors (e.g., BER₂). FFCR processor 410 maydecrease index 525 based on a product of a predetermined constant (e.g.,G_(N)) multiplied by the first difference between first quantity and thesecond quantity of bit errors (e.g., where decrease index 525=index525−G_(N)*(BER₁−BER₂)).

FFCR processor 410 may determine whether the decreased index 525 is lessthan a minimum index 525. FFCR processor 410 may set the decreased index525 to the minimum index 525 based on a determination that the decreasedindex 525 is less than the minimum index 525. FFCR processor 410 maycontinue to perform the operation on the signal to recover the estimatedcarrier phase, in a manner similar to that described above with respectto blocks 610-640, based on the decreased index 525. In another exampleimplementation, FFCR processor 410 may process another signal,associated with a second polarization state (e.g., that is parallel tothe Y-axis, which may be orthogonal to the X-axis), in a manner similarto that described above with respect to blocks 605-670).

As yet further shown in FIG. 6, if the first quantity of bit errors isnot greater than the second quantity of bit errors (block 640—NO), thenprocess 600 may include processing the signal using the first tapweights (block 660). For example, FFCR processor 410 may determine thatthe first quantity of bit errors is not greater than the second quantityof bit errors based on the comparison between the first quantity of biterrors and the second quantity of bit errors. Based on the determinationthat the first quantity of bit errors is not greater than the secondquantity of bit errors, FFCR processor 410 may process the signal usingthe filter that is set up based on the first tap weights 520.

As still further shown in FIG. 6, process 600 may include increasing theindex value based on a difference between the first and secondquantities of bit errors (block 665) and setting the index value equalto a maximum index value if the increased index value is greater thanthe maximum index value (block 670). For example, FFCR processor 410 mayidentify a second difference between the first quantity of bit errors(e.g., BER₁) and the second quantity of bit errors (e.g., BER₂). FFCRprocessor 410 may increase index 525 based on a product of thepredetermined constant (e.g., G_(N)) multiplied by the second differencebetween first quantity and the second quantity of bit errors (e.g.,where increased index 525=index 525+G_(N)*(BER₂−BER₁)).

FFCR processor 410 may determine whether the increased index 525 isgreater than a maximum index 525. FFCR processor 410 may set theincreased index 525 equal to the maximum index 525 based on adetermination that the increased index 525 is greater than the maximumindex 525. FFCR processor 410 may continue to perform the operation onthe signal to recover the estimated carrier phase, in a manner similarto that described above in blocks 610-640, based on the increased index525. Additionally, or alternatively, FFCR processor 410 may processanother signal, associated with a second polarization state (e.g., thatis parallel to the Y-axis, which may be orthogonal to the X-axis) in amanner similar to that described above in blocks 605-670).

FIG. 7 is a diagram of an example contour plot 700 that illustrates alevel of performance of a carrier phase recovery operation on a signalaccording to an implementation described herein. As shown in FIG. 7,contour plot 700 may illustrate a manner in which a quantity of biterrors, associated with a signal, are reduced as a result of performinga carrier phase recovery operation on the signal in a manner similar tothat described in FIG. 6. Additionally, contour plot 700 may illustratea manner in which a carrier phase recovery operation, associated withpolarization averaging, which will be described in greater detail withrespect to FIG. 8, can reduce the quantity of bit errors associated withthe signal. Contour plot 700 may include an index-axis data item 705(hereinafter referred to as “index-axis 705”), an XY average index-axisdata item 710 (hereinafter referred to as “XY average-axis 710”), a biterror rate (BER) data item 715, a non-optimum Q-point data item 720(hereinafter referred to as non-optimum Q-point 720), and an optimumQ-point data item 725 (hereinafter referred to as “optimum Q-point725”). FIG. 7 illustrates a number of data items for explanatorypurposes. In another example implementation, contour plot 700 mayinclude additional data items, fewer data items, different data items,and/or differently arranged data items than are shown in FIG. 7.

Index-axis 705 may identify a range of values, associated with index525, that correspond to a filter used by FFCR processor 410 whenperforming a carrier phase recovery operation. XY average-axis 710 mayidentify another range of values associated with an XY averaging indexthat identifies a manner in optical phase associated with constituentpolarization components of a signal are to be averaged. Bit error rate(BER) data item 715 may represent a line of constant bit rate errorrelative to a range of values that correspond to index 525 identified byindex-axis 705 and/or another range of values that correspond to an XYaveraging index identified by XY average-axis 710. For example, a firstBER data item 715 (e.g., labeled as +80%) may correspond to a quantityof bit errors that is 80% greater than a minimum threshold. In anotherexample, another BER data items 715 (e.g., labeled as +20%) maycorrespond to another quantity of bit errors that is 20% greater thanthe minimum threshold.

Non-optimum Q-point data item 720 may identify a point (e.g., shown as asolid black circle), associated with contour plot 700, that correspondsto N-index 525 (e.g., identified by index-axis 705) and/or an XYaveraging index (e.g., identified by XY average-axis 710). Non-optimumQ-point 720 may represent a first index-axis 525 and/or a correspondingfirst XY averaging index value that were used, by FFCR processor 410, toprocess a signal associated with a first quantity of bit errors that aregreater than a threshold. Non-optimum Q-point 720 may, generally,represent an early period, of the carrier recovery optimizationoperation, before a quantity of bit errors, associated with the signal,have been reduced to the minimum threshold.

Optimum Q-point data item 725 may identify another point (e.g., shown asa white circle), associated with contour plot 700, that corresponds toN-index 525 (e.g., identified by index-axis 705) and/or an XY averagingindex (e.g., identified by XY average-axis 710). Optimum Q-point 725 mayrepresent a second index-axis 525 value and/or a corresponding second XYaveraging index value that were used, by FFCR processor 410, to processa signal associated with a second quantity of bit errors that correspondto the minimum threshold. Optimum Q-point 725 may, generally, representa later period, of the carrier phase recovery operation, when a quantityof bit errors, associated with the signal, have been reduced to theminimum threshold.

For example, as shown in contour plot 700, the phase noise operation mayhave been initially performed, on a signal, using a first index 525 ofapproximately four and/or a first XY average index of approximately four(e.g., as shown by non-optimum Q-point 720). A position, associated withnon-optimum Q-point 720, may correspond to a quantity of bit errors thatare 80% higher than the minimum threshold (e.g., based on +80% BER dataitem 715). As shown in contour plot 700, the carrier phase recoveryoperation may cause XY averaging index to decrease as shown by a changein position, of non-optimum Q-point 720, downward from a XY averagingindex of four to another XY averaging index of approximately 1.0 to 1.5(e.g., as shown by bracket 730). The reduction in the XY averaging indexmay correspond to a reduction in a quantity of weighted averagingbetween a first component (e.g., corresponding to an X-polarization)and/or a second component (e.g., corresponding to a Y-polarization) ofthe signal. The reduction in the quantity of averaging may correspond toa reduction in the quantity of bit errors, associated with the signal,based another position of non-optimal Q-point 720 that corresponds toone or more locations between +20% and +40% BER data items 715.

As also shown in contour plot 700, the carrier phase recovery operationmay cause index 525 to increase, as shown by a change in position ofnon-optimum Q-point 720 in a rightward direction, from approximatelyfour to approximately six (e.g., as shown by bracket 735). The increasein index 525 may also cause non-optimum Q-point 720 to change to optimumQ-point 725. The increase in index 525 may correspond to a reduction inthe quantity of bit errors, associated with the signal, based on aposition, associated with optimal Q-point 725, that is within 20% BERdata item 715.

FIG. 8 is a flowchart of an example process 800 for performing a carrierphase recovery operation according to an implementation describedherein. In one example implementation, process 800 may be performed byFFCR processor 410 and/or FEC processor 420. In another exampleimplementation, some or all of process 800 may be performed by a deviceor collection of devices separate from, or in combination with, FFCRprocessor 410 and/or FEC processor 420.

Assume, in the description below, that FFCR processor 410 has performeda carrier phase recovery operation, on components of a signal, in amanner similar to that described above with respect to blocks 605-670 inFIG. 6. The components of the signal may correspond to a first signal,associated with an X-polarization, and a second signal associated with aY-polarization (e.g., where the Y-polarization is orthogonal to theX-polarization). Assume further that FFCR processor 410 has generated arespective value, associated with index 525, for each signal as a resultof the operation.

As shown in FIG. 8, process 800 may include receiving an indication thata carrier phase recovery operation is to be performed on a signal (block805). For example, FFCR processor 410 may receive an indication that afirst index 525 has been identified for the first signal associated withan X-polarization and/or that a second index 525 has been identified forthe second signal, associated with a Y-polarization. Based on thedetermination that the first and/or second indexes 525 have beenidentified, FFCR processor 410 may obtain and a default XY averagingindex that has been predetermined by FFCR processor 410 and/or anoperator associated with FFCR processor 410.

As also shown in FIG. 8, process 800 may include setting a firstaveraging value that is greater than the XY averaging index (block 810)and processing the first signal and/or second signal, to generatecarrier recovered signals, based on the first averaging value (block815). For example, FFCR processor 410 may identify a first averagingvalue that is greater than the default XY averaging index by apredetermined quantity. The predetermined quantity may, in one example,be a constant that is greater than zero (e.g., 0.25, 0.5, 1.0, etc.). Inanother example, the predetermined quantity may be a portion (e.g., suchas a percentage) of the XY averaging index that is greater than zero(e.g., 10 percent, 20 percent, etc.).

FFCR processor 410 may, in a manner similar to that described above withrespect to FIG. 4, use the first averaging value to set a firstweighting parameter (e.g., A) and/or a second weighting parameter (e.g.,B). FFCR processor 410 may generate first and/or secondcarrier-recovered signals that correspond to an X-polarization and/or anY-polarization, respectively. The first and/or second carrier-recoveredsignals may be corrupted by some quantity of noise (e.g., N₁ and/or N₂,respectively).

As further shown in FIG. 8, process 800 may include obtaining quantitiesof bit errors, associated with first and second carrier-recoveredsignals (block 820). For example, FFCR processor 410 may send, to FECprocessor 420, the first and/or second carrier recovered signals toobtain a first quantity of bit errors (e.g., BER₁) associated with thefirst carrier-recovered signal and a second quantity of bit errors(e.g., BER₂) associated with the second carrier-recovered signal.

FEC processor 420 may receive the first and second carrier-recoveredsignals and may perform an operation to identify the first and secondquantities of bit errors. FEC processor 420 may, in one example,identify the quantities of bit errors prior to performing a forwarderror correction operation on the first and/or second carrier-recoveredsignals. FEC processor 420 may, in another example, identify thequantities of bit errors during a period of time when the forward errorcorrection is being performed (e.g., after a first forward errorcorrection iteration and/or another forward error correction iteration).FEC processor 420 may send the identified quantities of bit errors(e.g., BER₁ and/or BER₂), to FFCR processor 410. FFCR processor 410 mayreceive the first and second quantities of bit errors.

As yet further shown in FIG. 8, process 800 may include setting a secondaveraging value that is less than the XY averaging index (block 825) andprocessing the first and second signals to other carrier-recoveredsignals based on the second averaging value (block 830). For example,FFCR processor 410 may identify a second value that is less than thedefault XY averaging index by the predetermined quantity and/or anotherpredetermined quantity.

FFCR processor 410 may, in a manner similar to that described above withrespect to FIG. 4, use the second averaging value to set the weightingparameter (e.g., A) and/or the second weighting parameter (e.g., B).FFCR processor 410 may generate third and/or fourth carrier-recoveredsignals that correspond to the X-polarization and/or an Y-polarization,respectively. The third and/or fourth carrier-recovered signals may becorrupted by some quantity of noise (e.g., N₃ and/or N₄, respectively).

As still further shown in FIG. 8, process 800 may include obtaining aother quantities of bit errors, associated with third and fourthcarrier-recovered signals (block 835). For example, FFCR processor 410may send, to FEC processor 420, the third and/or fourthcarrier-recovered signals to obtain a third quantity of bit errors(e.g., BER₃) associated with the third carrier-recovered signal and/or afourth quantity of bit errors (e.g., BER₄) associated with the fourthcarrier-recovered signal.

FEC processor 420 may receive the third and/or fourth carrier-recoveredsignals and may, in manner similar to that described above with respectto block 820, perform an operation to identify the third and/or fourthquantities of bit errors. FEC processor 420 may send the identifiedquantity of bit errors (e.g., BER₃ and/or BER₄), to FFCR processor 410and FFCR processor 410 may receive the identified quantity of biterrors.

As also shown in FIG. 8, if the quantities of bit errors is greater thanthe other quantities of bit errors (block 840—YES), then process 800 mayinclude processing the first and second signals based on the secondaveraging value (block 845). For example, FFCR processor 410 may comparea first sum of the first and second quantities of bit error (e.g.,BER₁+BER₂) to a second sum of the third and fourth quantities of biterror (e.g., BER₃+BER₄) and may determine that the first sum is greaterthan the second sum. Based on the determination that the first sum isgreater than the second sum, FFCR processor 410 may process the firstand second signals based on the second averaging value.

As further shown in FIG. 8, process 800 may include decreasing the XYaveraging index based on a difference between the quantities and otherquantities of bit error (block 850) and setting the XY averaging indexvalue equal to a minimum value if the decreased XY averaging index isless than the minimum value (block 855). For example, FFCR processor 410may identify a first difference between the first sum of the first andsecond quantities of bit error (e.g., BER₁+BER₂, respectively) and thesecond sum of the third and fourth quantities of bit error (e.g.,BER₃+BER₄). FFCR processor 410 may decrease the XY averaging index basedon a product of a predetermined constant (e.g., G_(A)) multiplied by thefirst difference between the first sum and the second sum (e.g., wheredecreased XY averaging index≅XY averagingindex−G_(A)*((BER₁+BER₂)−(BER₃+BER₄))).

FFCR processor 410 may determine whether the decreased XY averagingindex is less than a minimum value. FFCR processor 410 may set thedecreased XY averaging index to the minimum value based on adetermination that the decreased XY averaging index is less than theminimum value. FFCR processor 410 may continue to perform the carrierphase recovery operation on the first and/or second signal, in a mannersimilar to that described above with respect to FIG. 6 and/or withrespect to blocks 810-840 based on the decreased XY averaging index.

As yet further shown in FIG. 8, if the quantities of bit error are notgreater than the other quantities of bit error (block 840—NO), thenprocess 800 may include processing the first and second signals based onthe first averaging value (block 860). For example, FFCR processor 410may determine that the first sum, of the first and second quantities ofbit error, is not greater than the second sum of the third and fourthquantities of bit error. Based on the determination that the first sumis not greater than the second sum, FFCR processor 410 may process thefirst and second signals based on the first averaging value.

As further shown in FIG. 8, process 800 may include increasing the XYaveraging index based on a difference between the quantities and otherquantities of bit error (block 865) and setting the XY averaging indexvalue equal to a maximum value if the increased XY averaging index isgreater than the maximum value (block 870). For example, FFCR processor410 may identify a difference between the first sum of the first andsecond quantities of bit error (e.g., BER₁+BER₂, respectively) and thesecond sum of the third and fourth quantities of bit error (e.g.,BER₃+BER₄). FFCR processor 410 may increase the XY averaging index basedon a product of the predetermined constant (e.g., G_(A)) multiplied bythe difference between first sum and the second sum (e.g., whereincreased XY averaging index≅XY averagingindex+G_(A)*((BER₃+BER₄)−(BER₂+BER₁))).

FFCR processor 410 may determine whether the increased XY averagingindex is greater than a maximum value. FFCR processor 410 may set theincreased XY averaging index to the maximum value based on adetermination that the increased XY averaging index is greater than themaximum value. FFCR processor 410 may continue to perform the carrierphase recovery operation on the first and/or second signal, in a mannersimilar to that described above with respect to FIG. 6 and/or withrespect to blocks 810-840 based on the increased XY averaging index.

Systems and/or methods, described herein, may include a technique forrecovering carrier phase, associated with a signal received from anoptical coherent receiver, based on a quantity of errors associated withthe signal. As described herein, a feedforward carrier recovery (FFCR)device may dynamically tune a filter while processing the signal.

The foregoing description provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise form disclosed. Modifications and variations are possible inlight of the above teachings or may be acquired from practice of theimplementations.

For example, while series of blocks have been described with regard toFIGS. 6 and 8, the order of the blocks may be changed in otherimplementations. Also, non-dependent blocks may be performed inparallel.

Furthermore, while the disclosed embodiments have been presented asgenerally suitable for use in an optical network, the systems andmethods disclosed herein are suitable for any fiber optic network, fibernetwork, fiber line, or link that includes one or more transmissionspans, amplifier spans, or hops.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of the implementations. In fact, manyof these features may be combined in ways not specifically recited inthe claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one otherclaim, the disclosure of the implementations includes each dependentclaim in combination with every other claim in the claim set.

It will be apparent that embodiments, as described herein, may beimplemented in many different forms of software, firmware, and hardwarein the embodiments illustrated in the figures. The actual software codeor specialized control hardware used to implement embodiments describedherein is not limiting of the embodiments. Thus, the operation andbehavior of the embodiments were described without reference to thespecific software code—it being understood that software and controlhardware may be designed to implement the embodiments based on thedescription herein.

No element, act, or instruction used in the present application shouldbe construed as critical or essential to the implementation unlessexplicitly described as such. Also, as used herein, the article “a” isintended to include one or more items. Where only one item is intended,the term “one” or similar language is used. Further, the phrase “basedon” is intended to mean “based, at least in part, on” unless explicitlystated otherwise.

1. A method performed by a device, the method comprising: receiving,from an optical receiver, a signal derived from a first optical signaland a second optical signal generated by a local oscillator, where thesignal includes a first component that is an in-phase component and asecond component that is a quadrature phase component; filtering thesignal, using a filter, set to one or more configurations, to obtain oneor more recovered signals, where each of the one or more recoveredsignals include a respective quantity of noise; performing forward errorcorrection, on the one or more recovered signals, to obtain one or morequantities of bit errors that correspond to the one or more recoveredsignals; and processing the signal using the filter set to a particularconfiguration, of the one or more configurations, that corresponds to alowest quantity of bit errors of the one or more quantities of biterror.
 2. The method of claim 1, where the filter corresponds to afeedforward carrier recovery component, associated with the device; andwhere filtering the signal to obtain one or more recovered signals,further includes: filtering the signal using the filter, set to a firstconfiguration of the one or more configurations, to obtain a firstrecovered signal, of the one or more recovered signals; performing theforward error correction, on the first recovered signal, to obtain afirst quantity of bit errors, of the one or more quantities of biterrors filtering the signal using the filter, set to a secondconfiguration of the one or more configurations, to obtain a secondrecovered signal, of the one or more recovered signals; performing theforward error correction, on the second recovered signal, to obtain asecond quantity of bit errors, of the one or more quantities of biterrors; and processing the signal using the filter set to the firstconfiguration when the first quantity of bit errors is less than thesecond quantity of bit errors.
 3. The method of claim 1, where thefilter corresponds to a feedforward carrier recovery component,associated with the device; and where filtering the signal to obtain oneor more recovered signals, further includes: filtering the signal usingthe filter, set to a first configuration of the one or moreconfigurations, to obtain a first recovered signal, of the one or morerecovered signals; determining a first difference between the firstrecovered signal and the first optical signal, where the firstdifference corresponds to a quantity of bit errors; filtering the signalusing the filter, set to a second configuration of the one or moreconfigurations, to obtain a second recovered signal, of the one or morerecovered signals; determining a second difference between the secondrecovered signal and the first optical signal, where the seconddifference corresponds to another quantity of bit errors; and selectingthe first configuration or the second configuration based on whether thefirst difference is greater than the second difference.
 4. The method ofclaim 1, where the first optical signal is associated with a firstpolarization and a second polarization, where the first polarization isorthogonal to the second polarization.
 5. The method of claim 1, furthercomprising: obtaining a first index value that corresponds to a firstconfiguration of the one or more configurations; setting a plurality oftap weights, associated with the filter, in a manner that corresponds tothe first configuration based on the first index value, where the firstindex value identifies which tap weights, of the plurality of tapweights, are to be used to sample the signal; and where the plurality oftap weights are used to sample one or more bits associated with thesignal; and filtering the signal, using the filter set to the firstconfiguration, to obtain a first recovered signal of the one or morerecovered signals.
 6. The method of claim 1, where performing forwarderror correction on the one or more recovered signals further includes:identifying the quantity of bit errors, associated with a recoveredsignal, of the one or more recovered signals, prior to correcting thebit errors associated with the recovered signal.
 7. The method of claim1, further comprising: identifying, an optical phase associated with thesignal, where the optical phase includes a first optical phaseassociated with a first polarization and a second optical phaseassociated with a second polarization, where the first polarization isorthogonal to the second polarization; generating a first average of theoptical phase based on a first index that identifies a manner in whichthe first optical phase and the second optical phase are to be averaged;filtering the signal to obtain a first recovered signal, of the one ormore recovered signals, that includes a first portion of the firstaverage of the optical phase, where the first recovered signalcorresponds to the first polarization; obtaining a first quantity of biterrors associated with the first recovered signal; filtering the signalto obtain a second recovered signal, of the one or more recoveredsignals, that includes a second portion of the first average of theoptical phase, where the second recovered signal corresponds to thesecond polarization; and obtaining a second quantity of bit errorsassociated with the second recovered signal.
 8. The method of claim 7,further comprising: generating a second average of the optical phasebased on a second index that identifies a different manner in which thefirst optical phase and the second optical phase are to be averaged;obtaining a third recovered signal, of the one or more recoveredsignals, that includes a first portion of the second average of theoptical phase, where the third recovered signal corresponds to the firstpolarization; obtaining a third quantity of bit errors associated withthe third recovered signal; obtaining a fourth recovered signal, of theone or more recovered signals, that includes a second portion of thesecond average of the optical phase, where the fourth recovered signalcorresponds to the second polarization; obtaining a fourth quantity ofbit errors associated with the third recovered signal.
 9. The method ofclaim 8, further comprising: determining whether a first sum of thefirst quantity of bit errors and the second quantity of bit errors isgreater than a second sum of the third quantity of bit errors and thefourth quantity of bit errors; process the signal based on the firstindex when the first sum is less than the second sum; and process thesignal based on the second index when the first sum is not less than thesecond sum.
 10. The method of claim 1, where filtering the signal toobtain the one or more recovered signals further includes: obtaining afirst index value, associated with a plurality of tap weights that areused to filter one or more bits associated with the signal, and wherethe first index value identifies which tap weights, of the plurality oftap weights, are to be used to filter the signal; setting at least onetap weight, of the plurality of tap weights, based on the first indexvalue; and filtering the signal using the at least one tap weight, wherefiltering the signal is performed using the filter, the filter beingassociated with a feedforward carrier recovery component, associatedwith the device.
 11. The method of claim 10, where filtering the signalto obtain the one or more recovered signals further includes: obtaininga second index value, associated with the plurality of tap weights,where the second index value is different than the first index value;setting at least one other tap weight, of the plurality of tap weights,based on the second index value; and filtering the signal using the atleast one other tap weight.
 12. The method of claim 1, where the signalis generated, by the optical receiver, as a result of processing thefirst optical signal received from an optical transmitter, and where theone or more recovered signals correspond to a difference in phasebetween a first phase, associated with the signal, and a second phase,that corresponds to a local oscillator associated with the device.
 13. Adevice comprising: one or more processors to: receive, from an opticalreceiver, a signal derived from a first optical signal combined with asecond optical signal that is generated by a local oscillator, obtain,from the signal, a first recovered signal using a filter that is set upin a first configuration, where the first recovered signal includes afirst quantity of noise, identify a first quantity of bit errorsassociated with the first recovered signal, obtain, from the signal, asecond recovered signal using the filter that is set up in a secondconfiguration, where the second recovered signal includes a secondquantity of noise, identify a second quantity of bit errors associatedwith the second recovered signal, and process the signal using thefilter set up in the first configuration or the second configurationbased on whether the first quantity of errors is greater than the secondquantity of errors.
 14. The device of claim 13, where the firstrecovered signal and the second recovered signal are associated with afirst polarization; and where the one or more processors are further to:obtain, from the signal, a third recovered signal and a fourth recoveredsignal that are associated with a second polarization, where the secondpolarization is orthogonal to the first polarization, identify a thirdquantity of bit errors or a fourth quantity of bit errors associatedwith the third recovered signal or the fourth recovered signal,respectively, and process the signal using the filter set up in aconfiguration used to generate the fourth signal when the third quantityof errors is greater than greater than the third quantity of errors. 15.The device of claim 13, where, when obtaining the first recoveredsignal, the one or more processors are further to: retrieve a firstindex value, the first index value corresponding to the firstconfiguration, of the filter, that is used to obtain the first recoveredsignal, configure the filter to conform with the first configurationthat corresponds to the first index value, and process the signal, usingthe first configuration, to generate the first recovered signal.
 16. Thedevice of claim 13, where, when obtaining the second recovered signal,the one or more processors are further to: obtain a second index value,the second index value corresponding to a second configuration of thefilter, configure the filter to conform with the second configurationthat corresponds to the second index value, and process the signal,using the second configuration, to generate the second recovered signal.17. The device of claim 13, where identifying the first quantity of biterrors and the second quantity of bit errors, the one more processorsare further to: perform a forward error correction operation on thefirst recovered signal, where the forward error correction operation isperformed based on one or more iterations, where each iteration, of theone or more iterations, reduces a quantity of errors detected in thefirst recovered signal, and identify the first quantity of bit errors,associated with the first recovered signal after performing a firstiteration, of the one or more iterations, of the forward errorcorrection operation.
 18. The device of claim 13, where the one or moreprocessors are further to: obtain a first index value associated withone or more weighting factors that are used to determine a manner inwhich to generate a first weighted average of the optical phaseassociated with the signal based on a first optical phase and a secondoptical phase associated with the signal, where the first optical phaseis associated with a first polarization, and where the second opticalphase is associated with a second polarization.
 19. The device of claim18, where the one or more processor are further to: generate a thirdrecovered signal, associated with the first polarization, where thethird recovered signal includes a first portion of the first weightedaverage of the first optical phase and the second optical phase,identify a third quantity of bit errors associated with the thirdrecovered signal, generate a fourth recovered signal, associated withthe second polarization, based on the first index value, where thefourth recovered signal includes a second portion of the weightedaverage of the first optical phase and the second optical phase, andidentify a fourth quantity of bit errors associated with the fourthrecovered signal.
 20. The device of claim 19, where the one or moreprocessor are further to: generate a fifth recovered signal, associatedwith the first polarization, where the fifth recovered signal includes afirst portion of the a second weighted average of the first opticalphase and the second optical phase, based on the second index value,identify a fifth quantity of bit errors associated with the fifthrecovered signal, generate a sixth recovered signal, associated with thesecond polarization, where the sixth recovered signal includes a secondportion of the second weighted average of the first optical phase andthe second optical phase, identify a sixth quantity of bit errorsassociated with fourth recovered signal, determine that a first sum ofthe third quantity of bit errors and the fourth quantity of bit errorsis greater than a second sum of the fifth quantity of bit errors and thesixth quantity of bit errors, and process the signal based on the secondindex when the first sum is greater than the second SUM.
 21. A systemcomprising: one or more devices to: receive a signal, derived from afirst optical signal and a second optical signal, where the signalincludes a first signal associated with a first polarization and asecond signal, associated with a second polarization, that is orthogonalto the first polarization, retrieve a first index to generate a firstweighted average, of optical phase, associated with the first signal anda second weighted average, of the optical phase, associated with thesecond signal, filter the first signal and the second signal, using afilter that is set up based on the first weighted average, to generate afirst recovered signal, and the second weighted average to generate asecond recovered signal, obtain a first and second bit error rateassociated with the first recovered signal and the second recoveredsignal, respectively, retrieve a second index to generate a thirdweighted average, of the optical phase, associated with the first signaland a fourth weighted average, of the optical phase, associated with thesecond signal, filter the first signal and the second signal, using thefilter that is set up based on the third weighted average, to generate athird recovered signal, and the fourth weighted average to generate thefourth recovered signal, obtain a third and fourth bit error rateassociated with the third recovered signal and the fourth recoveredsignal, respectively, and process the first and second signals using thefilter set to the first configuration or the second configuration basedon whether a sum of the first bit error rate and the second bit errorrate is greater than another sum of the third bit error rate and thefourth bit error rate.
 22. The system of claim 21, where the firstoptical signal is transmitted by an optical transmitter via an opticalpath and where the second optical signal is generated by a localoscillator, where the first signal is a digital electrical signal thatincludes an in-phase component and a quadrature component, and where thesecond signal is another digital electrical signal that includes adifferent in-phase component and a different quadrature component. 23.The system of claim 21, where, when retrieving the second index togenerate the third weighted average and the fourth weighted average, theone or more devices are further to: identify a first parameter and asecond parameter, based on the first index, generate the first weightedaverage based on a sum of a first optical phase, associated with thefirst signal, multiplied by the first parameter and a second opticalphase, associated with the second signal, multiplied by the secondparameter, and generate the second weighted average phase based on a sumof the first optical phase multiplied by the second parameter and thesecond optical phase multiplied by the first parameter.
 24. The systemof claim 23, further comprising: set the filter to a first configurationthat is based on the first parameter and the second parameter, processthe first signal using the filter that is set to the firstconfiguration, to generate the first recovered signal, set the filter toa second configuration that is based on the first parameter and thesecond parameter, and process the second signal using the filter that isset to the second configuration, to generate the second recoveredsignal.
 25. The system of claim 23, where the first index or the secondindex specifies a level of correlation, between the first optical phaseand the second optical phase, that is to be used to process between thefirst signal and the second signal.